1. Field of the Invention
The present invention relates to a semi-transmissive Liquid crystal display device (LCD) that can improve display performance further while inhibiting an increase in dissipation power.
2. Description of the Related Art
As an LCD that combines functions of a transmissive LCD and those of a reflective LCD, a semi-transmissive LCD is known which type has a transmissive region and a reflective region in each of pixels. The transmissive region transmits light from a backlight light source used as a display light source and so can give good display in a relatively dark environment such as an indoor or dark room. The reflective region has a reflector plate so that external light reflected by this reflector plate may serve as a display light source and so can give good display in a relatively bright light environment such as the outdoors.
The LCD mounted in cellular phones and Personal Digital Assistances (PDAs) should preferably be semi-transmissive because it is used in a variety of environments covering indoor and outdoor environments. Further, the semi-transmissive LCD is capable of switching off the backlight light source as necessary and can save on dissipation power.
It is to be noted that LCDs has lateral field modes such as an In Plane Switching (IPS) mode and a Field Fringe Switching (FFS) mode. The lateral field mode LCD has a pixel electrode and a common electrode formed on the same substrate so that a lateral field may be applied to a liquid crystal layer. This lateral field mode LCD displays an image by turning liquid crystal molecules into a direction parallel with the substrate, thereby enabling realizing a wide viewing angle as compared to a Twisted Nematic (TN) mode LCD.
An LCD that combines the lateral field mode and the semi-transmissiveness is also developed and proposed.
In the lateral field mode semi-transmissive LCD, if its transmissive region is driven in a normally black mode, the reflective region is driven in a normally white mode, so that the transmissive region and the reflective region are opposite of each other in terms of light and darkness. Technologies to prevent the light-dark reversal are also under development. The related arts are described in, for example, Japanese Patent Application Publications, No. 2003-344837 (hereinafter, referred to as Related Art Patent Document 1) and No. 2005-338256 (hereinafter, referred to as Related Art Patent Document 2). The Related Art Patent Document 1 describes a configuration of the lateral field mode LCD in which an embedded retardation layer is disposed in the reflective region and the Related Art Patent Document 2 describes a configuration of the lateral field mode LCD in which a retardation film is disposed all over the transmissive region and the reflective region.
If the embedded retardation layer described in the Related Art Patent Document 1 is used, a problem occurs in that manufacturing steps for forming this embedded retardation layer will be increased greatly. Moreover, another problem occurs in that wavelength dispersiveness of the embedded retardation layer will disable realizing complete black when the reflective region is in the black display condition and so deteriorate contrast. On the other hand, if the retardation film described in the Related Art Patent Document 2 is used, a problem occurs in that in the transmissive region, the liquid crystal layer will have a smaller gap margin to resultantly decrease contrast and a viewing angle against changes in gap of the liquid crystal layer, thereby disabling sufficiently obtaining high contrast and wide-viewing angle characteristics in the lateral field mode.
Besides those, such technologies are proposed as to prevent light-dark reversal by improving the driving method without using the retardation film or the embedded retardation layer in the lateral field mode semi-transmissive LCDs (for example, Japanese Patent Application Publications, No. 2007-127933 (hereinafter, referred to as Related Art Patent Documents 3) and No. 2007-041572 (hereinafter, referred to as Related Art Patent Documents 4).
In these types of semi-transmissive LCDs, the transmissive region is driven in the normally black mode and the reflective region is driven in the normally white mode. The improved driving method mainly features that common electrodes (facing each other) are formed for the transmissive region and the reflective region independently of each other. Those common electrodes are supplied with common signals (reference voltages) different from each other so that different potentials can be applied to those common electrodes, thereby preventing light-dark reversal. Those setups enable obtaining high contrast and wide-viewing angle characteristics in the transmissive region in the lateral field mode and, further, realizing a lateral field mode semi-transmissive LCD that need not increase the manufacturing steps significantly.
However, there are a few problems in lateral field mode LCDs of the Related Art Patent Documents 3 and 4 in which a driving method is improved.
According to the Related Art Patent Document 3, in each sub-pixel, a planar common electrode is divided into two portions for the transmissive region and the reflective region respectively so that different reference voltages may be applied to the common electrode portion for the transmissive region and that for the reflective region.
When special attention is paid to, for example, the n-th display line, in each of the sub-pixels on the n-th display line, the common electrode in the reflective region is supplied with a HIGH level reference voltage and that in the transmissive region is supplied with a LOW level reference voltage. Further, the common electrode in the transmissive region aligned in the n-th display line is commonly used as the common electrode in the reflective region aligned in the (n+1)-th display line. When the n-th display line is being scanned, a potential somewhere between the LOW and HIGH levels is written to the pixel electrode, so that the pixels on the n-th display line have the same polarity. The polarity refers to the level of a potential at the pixel electrode with respect to that at the facing electrode, so that the polarity is positive if the pixel electrode has a higher potential than the common electrode and negative if it is reverse.
On the (n+1)-th display line, the common electrode in the reflective region is supplied with the LOW level reference voltage, so that the polarity at the reflective region on the (n+1)-th display line is opposite of that on the n-th reflective region. Similarly, the polarity at the transmissive region is opposite between the neighboring display lines.
As may be clear from this explanation, the driving method according to the Related Art Patent Document 3 involves gate line reversal driving, which driving deteriorates display qualities because it gives rise to lateral unevenness and flickering, so that dot reversal driving must be employed which gives an excellent display quality.
Further, since each sub-pixel has two common electrodes, to prevent short-circuiting, it is necessary to increase the distance between the common electrode for the transmissive region and that for the reflective region. This distance has an influence on the magnitude of a numerical aperture, especially if a pitch between the unit pixels is reduced. Accordingly, it is necessary to integrate the common electrode required into one which is common to the transmissive region and the reflective region in order to inhibit a decrease in numerical aperture, that is, display luminance.
According to the Related Art Patent Document 4, similar to the Related Art Patent Document 3, a common electrode is disposed to each of the transmissive region and the reflective region so that different common signals may be applied to those different common electrodes. Since the pixel electrode is supplied with a potential somewhere between the LOW and HIGH levels of the common signal, so that similar to the Related Art Patent Document 3, this approach also involves gate line reversal driving.
However, the Related Art Patent Document 4 describes a means configured to change the driving method from gate line reversal to dot reversal excellent in display quality and a means configured to integrate a common electrode required into one which is common to the transmissive region and the reflective region.
The means configured to change the driving scheme to dot reversal is to set a signal applied to the common electrode to a fixed potential and provide scanning lines for both the transmissive region and the reflective region. In accordance with periods during which the scanning line signals for the transmissive region and the reflective region are selected, the potential of an image signal that provides an electrical potential to be written to the pixel electrode is changed in each of the selected periods. This setup enables changing the polarity of the potential of the image signal for each of the vertical display lines, thereby performing dot reversal driving and also reducing count of the common electrodes required to one.
The other means configured to reduce count of the common electrodes required to one is to provide scanning lines for both the transmissive region and the reflective region, permitting the potential of the common electrode to change not in one horizontal period but in half this period. That is, the potential of the common electrode is held at the HIGH level during a period when the scanning line signal for the transmissive region is selected and at the LOW level during a period when the scanning line signal for the reflective region is selected. This setup enables the potential of the common electrode signal to be different between the transmissive region and the reflective region when writing the image signal potential to the pixel electrode, thereby integrating the common electrodes required into one which is common to the transmissive region and the reflective region.
However, even those tentative schemes have the following problems. According to the means configured to change the driving method from gate line reversal to dot reversal, the potential of the common electrode is fixed. Accordingly, it is necessary to enlarge the amplitude of the image signal, so that more electric power will be dissipated. On the other hand, according to the other means configured to integrate the common electrodes required into one which is common to the transmissive region and the reflective region, the signal applied to this common electrode needs an increased drive frequency, so that more electric power will be dissipated all the same.